1. Technical Field
The disclosure contained herein generally relates to high-voltage circuit breakers, and particularly to circuit breakers with gas-insulated tanks.
2. Description of the Related Art
Gas-insulated circuit breakers are known. Such circuit breakers generally include separable contact elements contained within a sealed tank filled with an inert insulating gas for reducing arcing. The gas is typically sulfur hexaflouride (SF6) due to its good insulative and arc interruption properties. The gas system for a circuit interrupter is typically a two-pressure closed cycle arrangement utilizing an efficient dielectric insulating gas such as SF6 gas, alone or in combination with other gases such as nitrogen or tetrafluoromethane. The high pressure gas is provided for the purpose of effecting arc extinction between the separable contacts of the circuit breaker. As a current interrupting medium, SF6 enables the extinction of high current arcs across the circuit-breaker contacts so as to accomplish the switching function, which is the main purpose of high voltage circuit breakers. Additionally, the low pressure gas inherently found within the enclosure provides the required dielectric insulation between the live or energized components within the grounded enclosure. The gas may be contained at a high pressure within a holding chamber, generally constructed of non-conducting material such as fiberglass, located within the enclosure.
Extinction of the arcs drawn between the contacts of the interrupter in the arcing area at the axial end of the movable contacts is aided by means of a blast of high pressure gas to the arcing area. The blast of high pressure gas may be released by operation of a blast valve. The opening operation of the blast valve may be synchronized with the opening of the contacts and may be accomplished by an associated linkage. A description of an exemplary blast valve, contacts and operating linkage may be found in U.S. Pat. No. 3,852,548, the disclosure of which is incorporated herein by reference in its entirety.
Gas type circuit breakers include puffer-type circuit interrupters and gas-blast interrupters. Puffer-type breakers generally comprise a pneumatic self-blast device which blows dielectric gas in the direction of the arc at the time of opening. A self-blast or puffer of this kind conventionally comprises a compression chamber including a piston coupled to a mobile contract of the circuit-breaker to move with it and adapted to blow a constant volume of cool gas in the direction of the breaking space during each opening. Gas-blast interrupters generally comprise the mechanically driven compression chamber and an additional chamber, located in between the compression chamber and the breaking space. The additional chamber is a thermal blast chamber wherein the gas is heated by the electrical arc. This gas is blasted onto the breaking space during interruption.
Other examples of blast mechanisms include the circuit breakers described in U.S. Pat. Nos. 4,650,941; 6,307,172; and 6,744,001, the disclosures of each of which are herein incorporated by reference in their entirety. In U.S. Pat. No. 4,650,941, a compressed gas high tension circuit breaker is described that uses a differential piston mechanism to separate the contacts and thus interrupt the circuit. U.S. Pat. No. 6,307,172 describes a high voltage circuit breaker with a gas-insulated tank enclosing separable contacts, which also includes a particle trap design. U.S. Pat. No. 6,744,001 describes a high voltage circuit breaker which includes a valve for decompressing a thermal belt chamber to deliver a dielectric gas to the breaking space for circuit interruption. Means of moving the separable contacts may include, for example, pneumatic and hydraulic systems, cam-spring systems, etc.
In circuit breakers of this type, SF6 gas is typically used as both an electrical insulating medium for high voltage components and also as a current interrupting medium. SF6 as an electrical insulating medium allows for reduced gaps between high voltage components and ground potential surfaces. SF6 gas as a current interrupting medium enables the extinction of high current arcs across the circuit-breaker contacts so as to accomplish the switching function, which is the main purpose of high voltage circuit breakers.
SF6 gas can be used alone or as a component in gas mixtures of SF6 and other gases. The density of the gas so employed is such that, at temperatures lower than some usual level (negative 30 degrees Centigrade, for example), liquefaction of the gas occurs and this diminishes the effective density of the remaining gas still in the gaseous physical state. This effect can create problems in some locations. For example, in some locations in North Dakota, Wisconsin, Minnesota and elsewhere, temperatures can drop well below −30° C.
Because of this low temperature limit that is dictated by physical laws of the gas, in low temperature locations, heaters are sometimes used to raise the temperature of the gas and therefore maintain appropriate densities in the gaseous state. Prior art designs use various heating configurations external to the metal enclosure of the high voltage circuit breaker, so as to achieve this heating of the gas. Such configurations include “blanket heaters” installed around the external surface of the tank, “box-type” heaters mounted directly on the tank wall, and other heating element types mounted in some way to accomplish heating of the gas. These external heater configurations have been combined with external thermal insulation to improve the overall efficiency of the gas heating function. Examples of such heaters are disclosed in, for example, U.S. Pat. No. 6,147,333 and International Patent Application No. WO 2002103734, the disclosures of each of which are incorporated herein by reference.
The external heater designs suffer from limited heating efficiency, as the heat that is ultimately delivered to the gas must pass through the tank wall before reaching the gas. Meanwhile, losses to the external environment are significant because of the external location of the heating element, and the temperature differential maintained between the heater element and that internal gas needed to accomplish significant heat transfer to that gas.
External heater designs are typically difficult to install or to exchange due to the need to maintain intimate contact with the external tank surfaces over the entire heating element area. Because of the placement of prior art heaters, hot spots in the heating element may develop, and heater failure can and does occur. As circuit breaker enclosures (tanks) are typically not perfectly round or flat, maintaining this intimate contact over the entire heating area is problematic. Further, temperature variations resulting from the external environment or from the heater operation may vary the dimensions of the tank and heater, alter material properties of the components of the enclosure, and accordingly modify heat transfer characteristics within the overall heater design. Solutions to this problem might include use of glues between the heater element and the external tank surface, or “belly-bands” to hold and compress the heater element onto the tank, or both.
Another limitation of typical external heater designs is that the heat is applied for a relatively short portion of the enclosure length, usually in the mid-section of a tank due to geometric constraints of the tank shape. This creates a very non-uniform temperature distribution in the gas in cold weather situations, the ends being much colder than the middle of the enclosure. Since pressure inside the gas volume is constant, density varies directly with temperature and the gas density varies significantly throughout the gas volume of the enclosure. This tends to actually reduce the gas density in the mid-section where the interrupters of the circuit breaker normally are located and thus reduce the electrical insulation and the current interruption capacity of the interrupter. Correct monitoring of the gas density at the interrupter (usually done by measuring temperature and pressure) is also quite difficult due to the large variations in temperature and gas density throughout the enclosure. Monitoring is often non-conservative, allowing operation of the circuit breaker even though the actual capacity is well below limits specified by required standards. Failure of the circuit breaker may result.
It is also desirable that circuit breaker designs allow for the control of conductive particles. An assembled circuit breaker tank can contain undesirable foreign particles, such as dust and metal shavings from machined parts. Additionally, when an arc exists across the contacts, heating of the contacts may result in the creation of conductive particles. It is undesirable for such particles, particularly metallic conductive or semiconductive particles, to freely reside within the tank. Such particles, if permitted to remain free, can interfere with the operation of the circuit breaker, causing undue arcing, flashing or promoting breakdown between metallic components. The presence of particles greatly reduces the breakdown voltage of the circuit breaker. Although unlikely, it is also possible that hardware such as nuts, washers, screws, etc., could work loose during operation. Sensitivity to particles increases with the voltage across the circuit breaker due to the increased electric field stress levels. Circuit breakers are now constructed capable of handling very high voltages, for example 362 kV and higher.
In a conventional tank, the operation of the contacts can cause such particles to move about the tank. For example, it is known that the operation of opening and closing the contacts causes shocks and vibrations capable of moving loose particles within the tank. Also, in a “puffer” type circuit breaker, the operation of opening the contacts results in flows of SF6 gas capable of blowing loose particles around the tank. Traps for foreign particles have been proposed in circuit breakers, such as the design seen in U.S. Pat. No. 6,307,172, the disclosure of which is incorporated herein by reference.
It is therefore desirable provide a circuit breaker with an improved means for maintaining the gas density and temperature throughout the interior chamber. The disclosure contained below is directed to solving one or more of the above-described problems.